Loaded-Board Test Fixture
After printed circuit boards (PCB's) have been manufactured and loaded with components, and before they can be used or placed into assembled products, they should be tested to verify that all required electrical connections have been properly completed and that all necessary electrical components have been attached or mounted to the board in proper position and with proper orientation. Other reasons for testing printed circuit boards are to determine and verify whether the proper components have been used and whether they are of the proper value. It is also necessary to determine whether each component performs properly (i.e., in accordance with the specification). Some electrical components and electromechanical components also may require adjustment after installation.
Loaded-board testing has complex multiplexed tester resources and is capable of probing soldered leads, vias and testpads on loaded boards with topside and bottom side components. Loaded-board testing includes analog and digital tests, such as tests for electrical connectivity, voltage, resistance, capacitance, inductance, circuit function, device function, polarity, vector testing, vectorless testing, and circuit functional testing. Loaded-board testing requires very low contact resistance between the test targets and the fixture components.
Advances in circuit board and electronic component packaging technology have escalated the probe spacing demands placed on loaded-board test equipment. Existing state-of-the-art technology requires loaded-board test equipment capable of accessing test targets which are spaced apart by 50 mils (center to center) or less, where test targets are physical features on a PCB or electronic component which may be probed during testing. One of the greatest challenges faced by loaded-board test equipment manufacturers now and in the future is a high false failure and test malfunction rate caused by physical and electrical contact problems. These problems are exacerbated by existing fixture limitations in probing accuracy, probing pitch (center to center spacing), and surface contamination.
As component and board geometries shrink and become denser, loaded-board testing becomes more difficult using standard fixtures. Existing shortwire, loaded-board fixtures can consistently hit test targets equal to or greater than 35 mils in diameter with equal to or greater than 75-mil pitch. Targets which are smaller or more closely spaced cannot be probed with consistency due to prohibitive component and system tolerance stack-ups.
A variety of test fixtures have heretofore been available for testing loaded boards on test equipment. A device under test (DUT) typically embodies a PCB loaded with electronic components and electronic hardware. FIG. 1 shows a conventional shortwire, loaded-board fixture, which consists of a DUT 108 with outer-layer artwork, a standard 106 or variable 118 tooling pin for alignment, a probe protection plate 104, standard spring probes 120 whose tips 116 exactly correspond to test target locations 110 and 112, spacers 114 to limit the deflection of the DUT under vacuum loading, a probe-mounting plate 102 in which the spring probes 120 are installed, personality pins 100 which are wired to the spring probes 120, and an alignment plate 122 which aligns the wirewrap tails of the personality pins 100 into a regularly spaced pattern so that they can line up with interface probes 124 mounted in the tester (not shown). Note: a spring probe is a standard device, commonly used by the test community, which conducts electrical signals and contains a compression spring and plunger that move relative to the barrel and/or socket when actuated. A solid probe also conducts electrical signals but has no additional parts which move relative to each other during actuation.
During test, the DUT 108 is pulled down by a vacuum or other known mechanical means to contact the tips 116 of the spring probes 120. The sockets of the standard spring probes 120 are wired to personality pins 100, and an alignment plate 122 funnels the long, flexible personality pin tips 126 into a regularly spaced pattern. The tips 126 of personality pins 100 contact the interface probes 124 located in the tester (not shown). Once electrical contact between the DUT 108 and the tester is established, in-circuit or functional testing may commence. Hewlett-Packard Company Application Note 340-1 titled "Reducing Fixture-Induced Test Failures," (printed December 1990 and can be obtained from Hewlett-Packard Company in Palo Alto, Calif.), discloses shortwire fixturing and is incorporated herein for all that it teaches. U.S. Pate. No. 4,771,234 titled "Vacuum-Actuated Test Fixture" by Cook et al. discloses a longwire fixture and is incorporated herein for all that it teaches.
FIG. 2 shows one conventional fixture that attempts to address limited-access problems during testing. The term "limited-access" refers to something that cannot easily be reached, or accessed, due to physical restrictions or constraints. For example, a limited-access PCB may contain many targets that are too closely spaced to accurately probe using existing fixture technology. The term "standard-access" refers to that which can be reached, or accessed, using existing fixture technology. The fixture of FIG. 2 consists of a DUT 206 with testpads 208 and 210, a tooling pin 204, a probe protection plate 202, standard spring probes 214 and 216 installed in a probe-mounting plate 200, and short probes 212 and 220 commonly referred to as "ULTRALIGN" probes (Ultralign is a registered trademark of TTI Testron, Inc.) installed directly in the probe protection plate 202. Upon actuation, standard spring probes 216 and 214 located in the probe-mounting plate 200 push against the floating plungers of "ULTRALIGN" probes 212 and 220. These short plungers are forced upward to contact test targets 208 and 210, while the sockets 218 and 222 remain fixed within the probe protection plate 202. An "ULTRALIGN" fixture may contain a mixture of spring probes for probing standard-access targets and "ULTRALIGN" probes for probing limited-access targets.
Despite its potential benefits, the "ULTRALIGN" fixture can be expensive and does not probe targets with a pitch of less than 50 mils. An "ULTRALIGN" fixture only permits limited probe travel which may result in poor connectivity between the probes 212 and 220 and the test targets 208 and 210. Also, these probes are costly and require expensive maintenance to replace worn or broken "ULTRALIGN" probes. An example of this type of fixture is disclosed in U.S. Pat. No. 5,510,722 entitled "Test Fixture for Printed Circuit Boards" to Seavey, which is incorporated herein for all that it teaches.
FIG. 3 shows a conventional guided-probe protection plate fixture. Guided-probe protection plates are used in standard loaded-board test fixtures to improve the pointing accuracy of spring probes. These plates contain cone-shaped through-holes which guide, or funnel, the tips of spring probes toward test targets. Such a fixture consists of a probe-mounting plate 300 with standard spring probes 312 and 314, a guided-probe protection plate 302 with spacers 310 and cone-shaped holes 316 for guiding the spring probes to the test targets 306 and 308 on the DUT 304. Additional manufacturing steps and increased fixture maintenance are required due to increased wear on the probes and the probe protection plate, and generally only narrow probe tip styles can be used. Although probing accuracy is slightly enhanced with this method, targets with center-to-center spacing of less than 75 mils cannot be probed reliably.
Bare-Board Test Fixtures
Bare-board testing probes testpads, vias, and plated through holes on bare printed circuit boards only and tests for electrical connectivity and continuity between various test points in the circuits on the printed circuit boards before any components are mounted on the board. A typical bare-board tester contains test electronics with a huge number of switches connecting test probes to corresponding test circuits in the electronic test analyzer.
While loaded-board testing can determine an electronic component's existence, proper orientation, or functionality, bare-board testing only checks for electrical continuity on PCB's without components. Bare-board testing does not require the very low contact resistance that loaded-board testing requires, nor does bare-board testing utilize sophisticated and complex multiplexed tester resources which must be assigned to specific targets and circuits on the device under test.
In previous years, PCB's were designed and manufactured so that their features resided in a regularly spaced pattern. During testing, the PCB was placed directly atop a regularly spaced pattern of interface probes located in the tester. As PCB and component geometries shrunk, PCB features could no longer be placed in a regularly spaced pattern and probed directly by interface probes. A bare-board fixture was developed which utilized long, leaning solid probes to provide electrical connections between small, closely spaced, randomly located targets on the PCB and regularly spaced interface probes located in the tester. Circuit C Inc. (Maple Grove, Minn.), Everett Charles Technologies (Pomona, Calif.), and Mania Testerion, Inc. (Santa Ana, Calif.), among others, make bare-board test fixtures which are commonly used on bare-board testers today.
Although each bare-board fixture builder uses unique components and manufacturing processes, most bare-board fixtures resemble FIG. 4 and include regularly spaced spring probes 414 on a tester and long, solid test probes 402 and 416 inserted through several layers of guide plates 400 drilled with small through-holes and held in a spaced-apart fashion with spacers 410. The bed of standard spring probes 414 actuate the solid test probes 402 and 416. The long, solid probes 402 and 416 may be inserted into the guide plates 400 vertically or at an angle in order to facilitate an easy transition between the fine-pitch, or very close, spacing of testpads 404 and 406 on the PCB side of the fixture and the larger-pitch spacing of the spring probes 414 on the tester side of the fixture. One such bare-board fixture is disclosed in U.S. Pat. No. 5,493,230 titled "Retention of Test Probes in Translator Fixtures" to Swart et al., which is incorporated herein for all that it teaches.
Existing bare-board fixtures can consistently hit test targets equal to or greater than 20 mils in diameter with equal to or greater than 20-mil pitch (center-to-center spacing). Unfortunately, it is not possible to use bare-board fixtures directly on a loaded-board tester because there are many unique features which render bare-board test equipment directly incompatible with loaded-board test equipment.
Bare-board fixtures are not designed to accommodate PCBs which are populated with electronic components; only PCB features which are flush with respect to the PCB (pads, vias, and plated through holes) can be probed. Bare-board testers are used to determine the connectivity and continuity of test points and circuitry in a PCB. Unlike bare-board testers, loaded-board testers cannot tolerate higher electrical resistance between a target on a PCB and the tester electronics. Loaded-board fixtures must provide low-resistance connections and interfaces between targets, fixture components, and tester electronics. Unlike loaded-board testers, bare-board testers cannot determine whether a component or a group of components exists and functions properly.
The spacing of bare-board tester interface probes is approximately 0.050 inches by 0.050 inches or 0.100 inches by 0.100 inches, while the spacing of Hewlett-Packard's tester interface probes is approximately 0.150 inches by 0.350 inches. The probe spacing of bare-board fixtures which are designed to fit on bare-board testers is not compatible with the interface probe spacing of Hewlett-Packard's loaded-board tester. Bare-board fixtures translate a target on the PCB under test to the nearest interface probe in the bare-board tester. However, loaded-board tester resources must be uniquely assigned and linked to specific targets and circuits. In loaded-board testing, the nearest interface probe may not be appropriate for a given target. Bare-board fixtures are not able to provide unique electrical routing to adjacent, nonadjacent, and remote tester resources; cannot reach remote resources; and cannot provide the complex, loaded-board resource routing patterns required by a loaded printed circuit board.
The term "no-clean" refers to the non-conductive solder flux residue which remains on printed circuit assemblies after components have been attached. Unless this contamination is removed, no-clean targets, or targets which are coated with this non-conductive surface residue, provide poor electrical contact and are difficult to test. Furthermore, industry trends, such as smaller component packaging and denser PCBs, are forcing electronics' manufacturers to confront smaller center-to-center target spacing, and small-diameter targets. These challenges require an improved loaded-board test fixture that is capable of providing reliable, consistent in-circuit and circuit functional testing of printed circuit assemblies by probing the smaller, more closely spaced targets on today's no-clean, loaded printed circuit boards, while at the same time probing vias and testpads on loaded-boards with top and bottom-side components and testing for electrical connectivity, voltage, resistance, capacitance, inductance, circuit function, device function, polarity, vector testing, vectorless testing, and circuit functional testing.
Loaded-board equipment manufacturers and fixture builders have designed several accessories and products to improve the testability of small, fine-pitch targets, but no design has completely solved the physical and electrical contact problems, while remaining competitively priced and easy to build and maintain. There is a need for such an improved loaded-board, guided-probed test fixture that solves physical and electrical problems related to limited-access testing, is competitively priced, accommodates the sophisticated resource assignments required by loaded-board testing, and is relatively easy and inexpensive to build and maintain. There is a further need for such an improved loaded-board, guided-probe test fixture that has improved probing accuracy, improved no-clean testability, and improved fine-pitch probing ability.